Certain properties of gypsum (calcium sulfate dihydrate) make it very popular for use in making industrial and building products; especially gypsum wallboard. It is a plentiful and generally inexpensive raw material which, through a process of dehydration and rehydration, can be cast, molded or otherwise formed into useful shapes. It is also noncombustible and relatively dimensionally stable when exposed to moisture. However, because it is a brittle, crystalline material which has relatively low tensile and flexural strength, its uses are typically limited to non-structural, non-load bearing and non-impact absorbing applications.
Gypsum wallboard; i.e. also known as plasterboard or drywall, consists of a rehydrated gypsum core sandwiched between multi-ply paper cover sheets, and is used largely for interior wall and ceiling applications. Because of the brittleness and low nail and screw holding properties of its gypsum core, conventional drywall by itself cannot support heavy appended loads or absorb significant impact. Accordingly, means to improve the tensile, flexural, nail and screw holding strength and impact resistance of gypsum plasters and building products have long been, and still are, earnestly sought. Another readily available and affordable material, which is also widely used in building products, is lignocellulosic material particularly in the form of wood and paper fibers. For example, in addition to lumber, particleboard, fiberboard, oriented strand board (OSB), plywood and hardboard (high density fiberboard) are some of the forms of processed lignocellulosic material products used in the building industry. Such materials have better tensile and flexural strength than gypsum. However, they are also generally higher in cost, have poor fire resistance and are frequently susceptible to swelling or warping when exposed to moisture Therefore, affordable means to improve upon these use-limiting properties of building products made from cellulosic material are also desired.
Previous attempts to combine the favorable properties of gypsum and cellulosic fibers, particularly wood fibers, have had very limited success. Attempts to add cellulosic fibers, (or other fibers for that matter), to gypsum plaster and/or plasterboard core have generally produced little or no strength enhancement because of the heretofore inability to achieve any significant bond between the fibers and the gypsum. U.S. Pat. Nos. 4,328,178; 4,239,716; 4,392,896 and 4,645,548 disclose recent examples where wood fibers or other natural fibers were mixed into a stucco (calcium sulfate hemihydrate) slurry to serve as reinforcers for a rehydrated gypsum board or the like.
U.S. Pat. No. 4,734,163 teaches a process in which raw or uncalcined gypsum is finely ground and wet mixed with 5-10% paper pulp. The mash is partially dewatered, formed into a cake and further dewatered by pressure rolls until the water/solids ratio is less than 0.4. The cake is cut into green boards, which, after being trimmed and cut, are stacked between double steel plates and put into an autoclave. The temperature in the autoclave is raised to about 140° C. to convert the gypsum to calcium sulfate alpha hemihydrate. During the subsequent incremental cooling of the boards, the hemihydrate rehydrates back to dihydrate (gypsum) and gives the board integrity. The boards are then dried and finished as necessary.
The process of water felting dilute aqueous dispersions of various fibrous materials is a well-known commercial process for manufacturing many types of paper and board products. In this process, an aqueous dispersion of fiber, binder and other ingredients, as desired or necessary is flowed onto a moving foraminous support wire, such as that of a Fourdrinier or Oliver mat forming machine, for dewatering. The dispersion may be first dewatered by gravity and then dewatered by vacuum suction means; the wet mat is then pressed to a specified thickness between rolls and the support wire to remove additional water. The pressed mat is then dried in heated convection or forced air drying ovens, and the dried material is cut to the desired dimensions.
U.S. Pat. No. 5,320,677 to Baig describes a composite material made from gypsum and cellulosic particles of a reinforcing material, such as lignocellulose fibers, hereinafter sometimes referred to as gypsum/wood fiber board. The composite material is produced by mixing gypsum and cellulosic particles of a stronger substance, such as wood fibers, in a dilute aqueous slurry. The slurry is heated in an autoclave, preferably under pressure, to convert the gypsum to calcium sulfate alpha hemihydrate. The hot, converted slurry is discharged through a headbox onto a continuous felting conveyor of the type used in paper making operations, where the slurry is dewatered to remove as much uncombined water as possible before rehydrating the hemihydrate back to gypsum. The resulting rehydrated material is a homogeneous mass comprising gypsum crystals physically interlocked with the discrete cellulosic particles. The resulting mat is then dried in heated convection or forced air drying ovens, and the dried board is cut to the desired dimensions.
In order to produce the best composite product it is essential to form acicular crystals of calcium sulfate alpha hemihydrate that have a fairly high aspect ratio because the crystals with high aspect ratios produce composite products with a high strength. However, if the crystals have an aspect ratio that is too high, the filter cake is difficult to dewater effectively. Further, problems occur if impurities, such as organic compounds, get into the gypsum/fiber slurry because some impurities can cause the calcination process to produce short blocky crystals that produce a GWF board that is low in strength. The crystals with the higher aspect ratios are smaller in diameter and have a greater surface area and it is theorized that the crystals with the higher aspect ratios provide a greater number of adhesion points as compared to the short blocky crystals. While the short blocky crystals may have higher tensile and flexural strength, they produce board products that are not as strong as the board products produced from the crystals with the higher aspect ratios.
Accordingly, one object of the present invention is to use selected crystal modifiers to reduce the time and/or lower the temperature conditions under which the calcium sulfate alpha hemihydrate crystals are formed. Another object of the present invention is to use crystal modifiers to produce a slurry containing calcium sulfate alpha hemihydrate crystals having aspect ratios within a selected target range.